Not an expert on these things but that's not my mental model of a hypervisor. What you're describing sounds more like an interrupt handler.... you know, a BIOS routine.
Generalizing massively, a simple hypervisor runs a small amount of code at boot to setup its own state somewhere, and injects its interrupt handlers into various places in the system, and finally chain boots the real OS. When the real OS tries to communicate with the underlying hardware, the hypervisor interrupt is triggered and it does it's stuff.
With modern virtualization acceleration, the hypervisor can enable and handle interrupts for an even wider range of events that it might need in order to efficiently interpose the OS.
A more full-featured hypervisor may start to look like a mini-OS with its own scheduler, etc. But a simple hypervisor is basically like an old DOS Terminate-and-Stay-Resident executable.
OK thanks. I imagined the distinction to be that a BIOS functions as an API and abstraction layer over the hardware, without preventing the OS from accessing it directly, whereas a hypervisor functions as an isolation layer, preventing an OS from direct access, and also thereby facilitating simultaneous hosting of multiple OSes on the same hardware.
